This invention relates to electronic commutation and more particularly to an improved method and apparatus for stopping current conduction through a thyristor in order to control the flow of direct current power in a desired manner.
To commutate a thyristor such as a silicon-controlled rectifier (SCR), that is, to switch it off when it has been conducting, the current passing through the SCR must be reduced below a critical value known as the holding current. The SCR does not, however, immediately regain its forward blocking characteristic and a short time must elapse before voltage is reapplied in the forward direction. This recovery time can be reduced by passing reverse current through the SCR during the switching off operation. There are a number of prior art SCR commutation circuits, most of which use inductive or capacitive circuitry to induce a counter current within the SCR to shut it off. These circuits suffer from various disadvantages such as high manufacturing costs, circuit complexity, unreliability, generation of electrical transients, and the generation of electrical stresses within the SCR. The above and other disadvantages of prior art SCR commutation circuits are overcome by the present invention.